multiboot_elfxx.c: Fix compilation by fixing undeclared variable

multiboot_elfxx.c: Fix compilation by fixing undeclared variable

Without that fix we have:
  In file included from ../../include/grub/command.h:25:0,
                   from ../../grub-core/loader/multiboot.c:30:
  ../../grub-core/loader/multiboot_elfxx.c: In function 'grub_multiboot_load_elf64':
  ../../grub-core/loader/multiboot_elfxx.c:130:28: error: 'relocatable' undeclared (first use in this function)
     "load_base_addr=0x%x\n", relocatable,

This happens due to mistake in the commit 14ec665
(mbi: Use per segment a separate relocator chunk).

So, let's fix it.

Signed-off-by: Ignat Korchagin <ignat@cloudflare.com>
Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@no-log.org>
Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>